When buying a French dictionary, you need to consider your language proficiency and what you'll be using the dictionary for. It's also important to keep in mind that bilingual dictionaries are a great tool, but they can have discrepancies, both major and minor. Their main weakness is in offering words that are no longer used. The following French-English / English-French dictionaries are arranged by quantity and quality of entries.
1. HarperCollins Robert French Unabridged Dictionary
This is the largest and best French-English English-French dictionary, with more than 2,000 pages. Entries include slang, regionalisms, and expressions. There's also a useful section on "language in use," with vocabulary and expressions grouped by categories such as suggestions, advice, business correspondence, and much more. In my opinion, this is the only option for fluent speakers and translators.2. HarperCollins Robert French College Dictionary
Abridged version of the above dictionary with 1,100 pages. Suitable for advanced students.
3. Larousse Concise French-English Dictionary
Paperback dictionary with 100,000 entries, including slang, culture, and more. Intermediate students will find that this dictionary has everything they need.4. Collins Gem French Dictionary
Very small, basic bilingual dictionary. Beginners and travelers can get by with it, but if they use it regularly, they'll soon realize this dictionary's limitations - it's only big enough for essentials.